Referring to FIG. 33, in a conventional immersion type membrane separator 1, a casing 2 is made up of a membrane case 3 and diffuser case 4. In the membrane case 3, a plurality of plate-like membrane cartridges 5 are so arranged that their membrane surfaces stand vertically at predetermined spaced intervals (usually, 6 mm to 10 mm). An air diffuser 6 is disposed in the diffuser case 4. In the plate-like membrane cartridge 5, a filtration membrane is attached to the front and rear surfaces of a filter supporting plate having rigidity, which is for example made of resin, and each filtration membrane is fused at its peripheral portion to the filter supporting plate by heating with a heater or by ultrasonic vibration. For example, the air diffuser 6 is obtained only by boring a plurality of blowholes of approximately φ10 mm in a lower part of a pipe made of vinyl chloride.
In the immersion type membrane separator 1, the air diffusing from the air diffuser 6 produces an upstream. An intra-tank mixed liquor in a treatment tank is supplied to a passage between adjacent plate-like membrane cartridges 5 by crossflow along the membrane surfaces and then subjected to crossflow filtration by the plate-like membrane cartridges 5. Permeated water that has been permeated through the filtration membranes is discharged to the outside of the tank via a tube 7 and collecting pipe 8.
Since an individual plate-like membrane cartridge 5 and collecting pipe 8 are connected via the tube 7 in the immersion type membrane separator 1, a large number of tubes 7 are present in the flow of the upstream, which constitute an obstruction to the flow.
On the other hand, during a period of time that the operation of the air diffuser is stopped, sludge enters a diffuser tube through the blowholes. This sludge is dried with air during air diffusion operation, and the dried sludge causes clogging of the blowholes and diffuser pipes. Clogging of some blowholes of the air diffuser 6 makes the upstream non-uniform, and some of the filtration membranes 5 fail to sufficiently receive membrane surface flushing effect. To prevent this, it is necessary to flush the air diffuser 6 in a periodic maintenance. This flushing is usually performed one time for two weeks. Letting water in the diffuser tube of the air diffuser 6 flushes the diffuser tube.
In the meantime, activated sludge process is employed as a method of treating organic wastewater having high organic and nitrogen concentrations. To maintain high activated sludge concentration in the tank, membrane separation activated sludge process using a membrane separator is employed, for example.
Referring to FIG. 32, in a water treatment system performing a general membrane separation activated sludge process, sundry matter is firstly removed from ale wastewater in a preprocessing facility 11 and then stored in a flow control tank 12. This is next supplied to a bioremediation tank (aeration tank) 13 at a fixed flow rate. In the bioremediation tank 13, organic materials in the wastewater is decomposed and removed by activated sludge. Then the resulting matter is subjected to liquid-solid separation in an immersion type membrane separator 14 immersed in the bioremediation tank 13, and then transferred to a sterilizing tank 16, through which the filtrate is discharged.
In this membrane separation activated sludge process, plate-like membrane cartridges 15 of the immersion type membrane separator 14 are disposed in a vertical multistage fashion in order to reduce the installation space of the bioremediation tank 13, thereby increasing the efficiency of filling of the membranes in the unit volume of the bioremediation tank 13. The basic construction of the immersion type membrane separator 14 and plate-like membrane cartridges 15 are the same as that in FIG. 33, and thus their description is omitted here.
Only liquid content of the intra-tank mixed liquor passing through the immersion type membrane separator 14 is removed by filtration, and the concentration of sludge is elevated. Therefore, the passage between the adjacent plate-like membrane cartridges 15 is located higher, the sludge concentration of the intra-tank mixed liquor is increased. This increases the filtration resistance of the intra-tank mixed liquor, and thus the passage between the membrane cartridges 15 may be clogged in some cases. Further, due to a reduction of the moisture content, the flow rate of the intra-tank mixed liquor decreases as the passage is located higher. As a result, the current velocity of the crossflow of the intra-tank mixed liquor passing between the plate-like membrane cartridges 15 is lowered and thus the membrane flushing effect is lowered.
When the amount of treatment is increased by arranging the plate-like membrane cartridges 15 of the immersion type membrane separator 14 in a vertical multistage fashion, the oxygen demand necessary for bioremediation in the tank 13 is increased to cause a deficiency of oxygen. Particularly, denitrification process requires a large oxygen demand. However, the air quantity diffused in the immersion type membrane separator 14 is limited, and thus excess air supply may lead to a drop in the efficiency of filtration, as well as damage to the membranes.
It is an object of the invention to overcome the foregoing problem by providing a multistage immersion type membrane separator capable of effecting a reliable production of crossflow and efficient filtration operation in such a construction of arranging plate-like membrane cartridges in a vertical multistage fashion, and a high-concentration wastewater treatment facility capable of effecting a sufficient oxygen supply when a multistage immersion type membrane separator is used in membrane separation activated sludge process.